A field effect transistor is widely used as a unit electronic device of a semiconductor memory integrated circuit, a high-frequency signal amplifier, a liquid crystal driving device or the like, and is an electronic device which is most widely put into practical use. Of these electronic devices, with development of a display in recent years, not only in a liquid crystal display (LCD), but also in various displays such as an electroluminescence display device (EL) and a field emission display (FED), a thin film transistor (TFT) has been widely used as a switching element which allows a display to be driven by applying a driving voltage to a display element.
In a TFT driving element, a silicon-based semiconductor thin film has been most widely used. On the other hand, due to superior stability to that of a silicon-based semiconductor thin film, a transparent oxide semiconductor thin film has attracted attention.
Oxide semiconductor films include an oxide semiconductor film containing a crystalline substance composed mainly of zinc oxide, and many studies have been made on this film. However, when a film is formed by a sputtering method, which is generally performed on the industrial basis, an oxide semiconductor film containing a crystalline substance mainly composed of zinc oxide encounters a problem that it tends to suffer oxygen deficiency, a large number of carrier electrons are generated, and electric conductivity is hard to be decreased. In addition, there is also a problem that abnormal discharge occurs when film is formed by a sputtering method, stable film formation cannot be attained, resulting in deterioration of homogeneity and reproducibility of the resulting film.
Further, an oxide semiconductor film containing a crystalline substance composed mainly of zinc oxide has a low field effect mobility (hereinafter, often referred to as the “mobility”) of about 1 cm2/V·sec. This film has a small on-off ratio and a tendency to occur current leakage easily. Therefore, when an oxide semiconductor film containing a crystalline substance composed mainly of zinc oxide is used as an active layer (channel layer) of a TFT, a large amount of current is flown between a source terminal and a drain terminal even when no gate voltage is applied, and as a result, a TFT cannot attain a normally-off operation. In addition, it is difficult to increase the on-off ratio of a transistor.
As mentioned above, a TFT obtained by using an oxide semiconductor film containing zinc oxide has disadvantages that TFT properties tend to be deteriorated, i.e. a low mobility, a low on-off ratio, a large amount of current leakage, an unclear pinch-off voltage, tendency of being normally-on easily, or the like. Further, the film has restrictions on production processes or operating environments due to difficulty in wet etching caused by poor chemical resistance.
As for an oxide semiconductor film containing a crystalline substance composed mainly of zinc oxide, film formation is required to be conducted at a high pressure in order to improve performance such as mobility. Therefore, the film deposition rate is low. Further, since a heat treatment at a high temperature of 700° C. or more is required, industrialization is difficult.
In addition, in the case of a TFT using an oxide semiconductor film containing a crystalline substance composed zinc oxide, TFT performance such as mobility is low in the case of a bottom-gate configuration. In order to improve performance, it is required to increase the film thickness to 100 nm or more in a top-gate configuration. Accordingly, the film also has a restriction on device configuration of a TFT.
In order to solve these problems, application of an amorphous oxide semiconductor film comprising indium oxide, gallium oxide and zinc oxide to thin film transistor is studied. In addition, studies are made on forming an amorphous oxide semiconductor film composed of indium oxide, gallium oxide and zinc oxide is formed by a sputtering method, which is a method that enables effective mass production on the industrial basis. Gallium is a rare metal and expensive, and use thereof leads to an increase in a raw material cost. However, if gallium is added in a small amount, a normally-off operation of a TFT cannot be realized as long as the oxygen partial pressure at the time of film formation is not increased.
On the other hand, a thin film transistor obtained by using an amorphous oxide semiconductor film which does not contain gallium and comprises indium oxide and zinc oxide is proposed (Patent Document 1 and Non-Patent Document 1). However, as mentioned above, as long as the oxygen partial pressure at the time of film formation is not increased, a normally-off operation of a TFT cannot be realized.
In addition, a sputtering target for use as a protecting layer of a light information storage medium obtained by adding elements such as Ta, Y and Si to an In2O3—SnO2—ZnO-based oxide which is composed mainly of tin oxide is studied (Patent Documents 2 and 3). This sputtering target has problems that it is not for use in an oxide semiconductor and an agglomerate of an insulating material tends to be formed easily, and hence, a resistance value is increased or abnormal discharge tends to occur easily.